The invention relates to a method and apparatus for compensating for changes in the response characteristic of a modulator particularly a beam modulator for use in an image generation system.
In conventional image generation systems, an original image is scanned to generate signals representing for example cyan, magenta, and yellow which, after processing, are fed to an exposing beam assembly which generates one or more exposing beams which are modulated in accordance with the scanned signals. The exposing beams impinge on a record medium which may be a light sensitive sheet or gravure cylinder. In the case of half-tone imaging, the control information is modified with half-tone dot information. Typically, the intensity of an exposing beam is controlled by a beam modulator.
In this context, a modulator comprises a device for controlling the transmission of radiation passing through the modulator in accordance with an applied control function, the response characteristic of the modulator to the control function exhibiting a maximum or minimum. Such a modulator is hereinafter referred to as of the kind described. Typically, the response characteristic will be `U`or `V`-shaped and conveniently is symmetrical about the maximum or minimum. In this context `maximum` refers to maximum radiation transmission and `minimum` to minimum transmission or extinction.
A typical beam modulator of the kind described comprises an electro-optical modulator which responds to an applied voltage. Typically, the response characteristic has a sin.sup.2 form, that is the intensity of light transmitted varies in a sin.sup.2 fashion in response to applied voltage.
In practice, the response characteristic of conventional modulators is not constant and tends to vary and, in some cases, drift over fairly short time periods due to temperature variations. Typical drifts are in the order of tens of volts. Where a number of such modulators are mounted together, such as in the Crosfield Magnascan 645 system, variations greater than about 2 volts in response characteristics can lead to the generation of moire patterns in some colour separations which is undesirable.
Various proposals have been made for compensating for this drift. U.S. Pat. No. 3,579,145 describes a system in which a pilot signal is superimposed upon the information bearing modulating signal. The resulting modulation produced by the pilot signal is compared with a reference signal in a phase detector and a control signal is generated which is used to counteract any drift of the operating point.
Similar approaches to that of the U.S. specification mentioned above are described in a paper entitled "Electronic Stabilisation of the Operating Point of an Electro-Optical Light Modulator with Low Control Voltages" in Radio Engineering and Electronic Physics, Vol 15, No. 9 pp 1659-1662 and in a paper entitled "Automatic Adjustment of the Position of the Operating Point of an electro-optical modulator" in Instruments and Experimental Techniques Vol 25, No. 5, part 2 pp. 1231-1233.
All these techniques work in real time. This is possible because they describe the information bearing modulating signal as a pulse code modulated (PCM) signal. In PCM a small change in amplitude is not relevant. However, these systems are not applicable in image generation systems where a low amplitude signal instead of a zero amplitude signal is very significant and undesirable.